1. Field of the Invention
The present invention relates to a digital signal magnetic recording/reproducing apparatus such as a digital video tape recorder (DVTR).
2. Description of the Prior Art
In conventional digital signal magnetic recording/reproducing apparatuses, due to the difficulty of recording/reproducing the direct current component, there have been used base band modulations having reduced direct current component in the recording signals, such as NRZ codes (e.g., J. K. R. Heitmann "An Anaitical Approach to the Standardization of Digital Video Tape Recorder", SMPTE J., 91,3, Mar. 1982, or J. K. R. Heitmann "Digital Video Recording, New Result in Channel Coding and Error Protection", SMPTE J., 93:140-144, Feb. 1984), 8-10 block codes (e.g. J. L. E. Baldwin "Digital Television Recording with Low Tape Consumption", SMPTE J., 88:490-492, July, 1979), Miller-square (M.sup.2) codes e.g., L. Gallo "Signal System Design for a Digital Video Recording System", SMPTE J. 86:749-756, Oct. 1977), ternary partial response system, etc.
However, in the digital signal magnetic recording/reproducing apparatuses utilizing such base band modulations, due to the basic system of recording the binary signals, utilization efficiency of the recording frequency band (transmissible bit rate per unit band) is low. For instance, assuming the roll-off rate of the nyquist transmission system (i.e. a transmission system having a role off spectrum free from intersymbol interference) to be 0.5, the utilization efficiency of the frequency band is at most 1.33 bits/sec./Hz. This incurs an increased tape consumption, making difficult long time recording. Further, in order to increase the recording rate, there can be considered a method of expanding the recording frequency band, a method of increasing the number of recording channels, or a method of increasing the relative velocity However, if the recording frequency band is expanded, the S/N ratio is substantially deteriorated. Accordingly, the recording rate cannot be increased to any great extent. When the number of recording channels is increased, the track width is narrowed and the S/N ratio is deteriorated. If the relative velocity is increased, the amount of tape consumption naturally comes into question.
As for the deterioration of the S/N ratio, the error rate may be improved by using the error control coding. However, the bit rate of the data which can be actually recorded is lost by such coding (Ref. L. M. H. E. Dreissen et al., "An Experimental Digital Video Recording System", IEEE Conf. Jun. 1986, or C. Yamamitsu et al., "An Experimental Digital VTR Capable of 12-hour Recording", IEEE Trans. on CE,CE-33, No.3, pp 240-248, 1987).
Further, there has been proposed a method of preventing the deterioration of the S/N ratio by frequency equalization in such manner that, during the detection of the reproduction signal, Viterbi decoding is performed by regarding the intersymbol interference as a convolutional coding. In this method, however, in the recording/reproducing of the binary pulse sequences, an improvement is realized only in the detection capacity, and the utilization efficiency of the frequency band still remains low (cf. K. Matsushita et al "path Feedback Viterbi Detection without Opening Eye", IEICE (the Institute of Electronics, Information and Communication Engineers of Japan) Report, MR87-38, Vol.87, No.265, pp. 21-27, 1988).
On the other hand, there has been proposed a coded modulation scheme carried out by combining a multi-value modulation scheme, which shows a high utilization efficiency of the frequency band, with an error control coding for application to the digital data transmission. This, however, does not give consideration to the special conditions such as the effect of non-linear distortion or the effect of saturation characteristic in the case of making recording/reproducing on a magnetic recording medium (cf. G. Ungerboeck "Channel Coding with Multilevel/phase Signals", IEEE Trans. on IT, IT-28, No. 1, pp. 55-67, 1982).
We have already proposed a new digital signal magnetic recording/reproducing apparatus having a high utilization efficiency of the frequency band and high recording rate in U.S. patent application Ser. No. 07/251,094 (filed on Sept. 29, 1988). In this apparatus, however, no consideration is given to the improvement of the error rate of the reproduced signal.